30 April 1999
Parasites, Not Pollution, Responsible For Some Frog Deformities
Frogs with multiple limbs and other disfigurements have been increasingly reported in recent years, fuelling speculation that the ecologically sensitive amphibians are indicators of some larger environmental catastrophe. But now two studies published in the latest issue of Science provide the strongest evidence yet that at least some of the frogs' bizarre deformities are the direct result not of pesticides or ozone loss, but of a parasite that naturally infects the animals.
Scientists first began noticing a global decline in amphibian populations about 15 years ago, and public concern increased throughout the 1990s. Speculation as to the cause has included possible DNA damage to frog embryos (as a result of too much UV light passing through the thinning ozone layer) and pollution from pesticide run-off. A strong candidate for the latter has been the powerful chemical known as retinoids. In amphibians-which regenerate lost limbs-retinoids can scramble the genetic information at the site of the new limb bud, resulting in what looks like multiple legs.
But in one of the studies, Stanley Sessions and colleagues provide firm evidence in support of another candidate: small parasitic flatworms called Riberoria trematodes. These creatures burrow into the hindquarters of tadpoles where they physically rearrange the limb bud cells and thereby interfere with limb development.
"It's about as close to using an egg beater on the limb bud cells as you can get," said Sessions.
Sessions and his colleagues drew upon earlier laboratory studies showing that burrowing parasites cause a different type of limb deformation than the chemical effects of retinoids. Retinoids turn a limb bud topsy-turvy, the most common result being that cells that would normally develop at the end of a limb grow at the base of the limb, and cells intended for the base develop at the tip. This results in a characteristic thigh-knee-thigh-knee pattern. In the case of trematode infections, however, two or three properly patterned limbs sprout from one limb bud, each the mirror image of the one next to it.
Taking these laboratory results into the field, Sessions and his colleagues studied five species of frogs from twelve different locations in the United States. They found none of the thigh-knee-thigh-knee deformities that are the exclusive signature of retinoids, and found many of the mirror-image deformities that are mostly characteristic of parasite infections.
While Sessions' team was sampling frogs from ponds around the country, another team of scientists was working inside the lab gathering more evidence in support of the trematode theory. Pieter Johnson and his colleagues exposed tadpoles to trematodes under carefully controlled conditions. The tadpoles grew into frogs with deformed hind legs that closely matched deformed frogs found in the wild. In contrast, tadpoles raised without any exposure to parasites did not have any deformity.
The two findings resolve scientists' concerns about amphibians, however. Other kinds of frog deformities are being reported, including missing or malformed limbs and eyes, which may be caused by something other than parasites. In fact, while multiple limbs may be the most striking type of deformity, they are not the most common.
Johnson and Sessions both think that the parasites may exploit the frogs' deformities as part of their own reproductive cycle. After hatching in pond water, the immature trematodes infect aquatic snails, where they reproduce asexually, increasing their numbers to the point that often kills the snail. Back in the water, the trematode larvae next seek out tadpoles and burrow into their hind leg regions. Later, aquatic birds prey on the grown frogs giving the parasites a third host, where they reproduce sexually. Finally, the eggs reach another pond via the birds' faeces, completing the cycle. Because birds prey upon them significantly more often than on normal frogs, deformed frogs may play an unfortunate but critical role in the parasite's reproduction strategy.
Although parasite infections appear to be the direct cause of multiple limb deformities, human activities may be exerting a less direct influence by affecting the populations of one of the trematode's other hosts. For example, fertiliser-filled run-off could be filling the ponds with excess nutrients, which in turn would boost the populations of the aquatic snails. More research is necessary to understand the natural fluctuation in trematode populations before scientists can draw any conclusions about human effects. Also, these deformities may not be on the increase at all, just more often reported by an alert public. In fact, Sessions speculates that parasite-induced infections may actually be a sign that all is well in the frog environment. Only robust cells in the limb bud can divide and form multiple limbs. This rigorous response to local infections, he says, may in fact suggest that the tadpoles are otherwise healthy.